Composite materials comprising ceramic and metallic phases, generally referred to as cermets, are known from numerous publications. Such materials are used in a wide variety of applications and their specific composition, structure and other physical and chemical properties may be adapted to the specific intended use.
Among the known types of cermets are those which comprise metal oxides as ceramic phase with a different metal or the same metal as the metallic phase. An example is given in EP-A-0 072 043 which describes a cermet comprising alumina and aluminum. This cermet is proposed for use as a component covered with molten aluminum in fused salt aluminum electrowinning cells. This reference also mentions (page 5, line 30) that the alumina phase may include further oxides which do not readily react with aluminum. The metallic phase may either be pure aluminum or an alloy of aluminum with other metals such as mentioned on page 3, lines 5-32.
It has already been proposed to include cerium compounds in ceramic materials used as components in aluminum production cells. Such developments are described e.g. in EP-A-0 115 689, which discloses a ceramic body fabricated by reaction sintering CrO.sub.3 and CeB.sub.2 to yield a mixture of ceria and CrB.sub.2.
U.S Pat. Nos. 4,454,015 and 4,584,172 have subsequently disclosed various reaction-sintered cermets such as Fe/Ni oxide as ceramic phase with iron, nickel or an iron/nickel alloy as the metallic phase. These cermets were proposed for dimensionally stable anodes in molten salt electrowinning.
EP-A-0 114 085 discloses aluminum electrowinning with a cerium compound dissolved in the molten cryolite electrolyte so that during electrolysis a protective coating of a fluorine-containing oxycompound of cerium referred to as "cerium oxyfluoride" is produced on the anode surface. The anode substrate to be plated with the cerium oxyfluoride was for example SnO.sub.2, but other substrates were also proposed such as electrically conductive and essentially corrosion resistant materials which, in addition, may contain or be precoated with cerium as metal, alloy or intermetallic compound with at least one other metal, or as compound. The substrate may in particular be a conductive ceramic, cermet, metal, alloy or an intermetallic compound.